Technical Abstract:
Non-typhoidal Salmonella strains cause ~1 million cases of foodborne disease each year in the U.S. and are a leading cause of food-related deaths. The prevalence of multidrug-resistant (MDR) Salmonella serovars has increased over the last few decades, and infection with these strains has an increased risk for morbidity and mortality. Salmonella frequently colonizes food-producing animals, and based on multiple studies including USDA NAHMS data, >50 % of swine production facilities in the U.S. test positive for Salmonella. Salmonella spp. often contain prophage that when induced may transfer antibiotic resistance or virulence genes to susceptible bacteria. MDR S. Typhimurium DT104 NCTC13348 contains five prophages in the DT104 genome that may be induced to produce phage under various environmental conditions, including antibiotic exposure. The antibiotic carbadox is frequently used in the U.S. during the starter phase of swine production for performance enhancement and control of enteric diseases. In the current study, we investigated whether carbadox exposure of MDR DT104 would induce phage production. A sub-inhibitory concentration of carbadox induced phage in DT104, resulting in bacterial cell lysis. Deletion of the Gifsy-1-like prophage from the DT104 genome resulted in decreased bacterial cell lysis following carbadox exposure, suggesting that the Gifsy-1-like prophage was induced in wild-type DT104. The transcriptional response of MDR DT104 following carbadox exposure was profiled using RNA-seq. Numerous genes from three (Gifsy-1-like, ST64B, and P22-like) of the five prophages had significantly increased expression following carbadox addition, suggesting that multiple prophages were induced concurrently and that a phage induction hierarchy may exist. Furthermore, utilizing a carbadox-induced, phage lysate from MDR DT104, we demonstrated genetic transfer of non-phage DNA to a recipient S. Typhimurium, including plasmid DNA conferring antibiotic resistance and Salmonella genomic DNA. Since P22 phage is known to facilitate generalized transduction, the P22-like prophage may be responsible for the gene transfer phenotype. Our research indicates that carbadox exposure of DT104 induces multiple prophage and demonstrates antibiotic-induced gene transfer from MDR S. Typhimurium DT104.